Geochronology and geochemical characteristics of volcanic rocks from the Manitu Formation in Qilibin area, northern Great Xing’an Range and its geological signaficance
Abstract:The volcanic rocks in Manitu Formation from the Qilibin area,northern Great Xing"an Range are mainly composed of andesite, hungarite,trachyandesite,but they are mainly trachyandesite with minor trachyte based on chemical composition. Zircon U-Pb dating results show that they formed during the Early Cretaceous with the age of 141±1.2 Ma. The geochemical characertistics of the rocks indicate that the volcanic rocks in Manitu Formation from Qilibin area are high in SiO2 (54.09%~64.89%), total alkaline K2O+Na2O (6.33%~8.74%), Al2O3(14.67%~17.27%) and low in MgO (0.81%~2.71%) and Mg# (24.84~46.15), belonging to the calc-alkaline series with high K contents; The rare earth elements analysis show that the total rare earth elements (∑REE) are in 146.51×10-6~193.29×10-6 with insignificant negative Eu anomalise (δEu=0.64~0.96) and intense fractionation between LREE and HREE ((La/Yb)N=8.5~15.52); The trace elements are characterized by LILE (e.g., Rb, Ba, K) and LREE enrichment and HFSE (e.g., Ta, Nb, Ti, Zr, P) depletion, meanwhile, they contain positive Hf anomalies, high ratios of Th/Ta, Th/Nb, low ratios of Lu/Yb (0.14~0.16) and Rb/Sr (0.02~0.17). Combining their geochemical characteristics with previous studies of this area, the magma for the volcanic rocks in Manitu Formation from Qilibin area was derived from the partial melting of the subduction fluid metasomatized mantle wedge, and experienced fractional crystallization and contaminations of crust during the magma ascent. It is possible that these volcanic rocks in Qilibin area were formed by the collision-postorogenic extension of Mongolia-Okhotsc ocean and the upwelling of asthenosphere.
